In recent years, vehicles equipped with battery packs have become popular. Examples of such vehicles include electric assist bicycles and electric automobiles. Battery packs provided to such vehicles typically include a secondary battery, and such battery packs are charged while the vehicles are not being used. Patent Literature 1 discloses a battery pack for an electric motorcycle or an electric assist bicycle.
Further, battery packs with built-in secondary batteries are useful for making effective use of small-sized power generation facilities for household use, which may use solar power or fuel cells. For example, effective use of such a power generation facility is possible by charging a battery pack with surplus power generated when power output from the facility is greater than power consumed by a load, and by discharging the battery pack when the power output from the facility is smaller than the power consumed by the load. Such form of use eliminates the need of wasting power or reducing the output from the facility when the power output from the facility is greater than the power consumed by the load, and also reduces the amount of power that needs to be purchased from a commercial power source when the power output from the facility is smaller than the power consumed by the load. Accordingly, such form of use prevents a waste of the power output from the facility.